Electro-optic devices using liquid crystals which have hitherto been developed and put into practical use include those using nematic liquid crystals, such as a DSM mode, a TN mode, a G-H mode, and an STN mode. However, any of these devices using nematic liquid crystals has a very slow electro-optic response requiring a switching time from several to several tens milliseconds and is hence limited in range of application. The slow response of the elements using nematic liquid crystals is because the torque of moving molecules, basically being based on anisotropy of dielectric constant, is not so strong. Under such circumstances, Meyer, et al. developed ferroelectric liquid crystals which undergo spontaneous polarization (Ps) and has a strong torque, the torque being based on Ps x E (applied electric field), and thereby has a high-speed response in the order of microseconds as disclosed in Le Journal de Physique, Vol. 36, L 69 (1975). Further, JP-A-63-307837 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses new ferroelectric liquid crystals, but gives no description about "three states" hereinafter described.
Several high-speed electro-optic devices using ferroelectric liquid crystals have already been proposed. Typically included in such devices is an element in which a twisted structure is untwisted by the force of a wall surfaces, and two molecular alignment layers in parallel in the wall surface are varied by polarity of the applied electric field as described, e.g., in JP-A-56-107216.
Existence of a compound showing ideal two states having an electric field response waveform as shown in FIG. 1 is prerequisite to realization of the above-described device. However, such a compound exhibiting ideal two states has not yet been discovered. Any of the so far synthesized bistable liquid crystals has a response waveform as shown in FIG. 2 but not that of FIG. 1. When the state-of-the-art liquid crystals having a response waveform of FIG. 2 are used, for example, in light switching circuits, since a transmission gradually changes according as an applied voltage changes from negative to positive, the purpose cannot be sufficiently accomplished simply by changing the applied voltage between "on" and "off". Moreover, so far synthesized bistable liquid crystals find difficulty in making a mono-domain state in its Sc* phase with no voltage applied, i.e., an ideal molecular orientation state, easily causing defect or a molecular orientation disturbance called twist. Thus, it has been difficult to realize the above-stated ideal two states of molecular orientation over a wide area. Further, because the threshold value (voltage at which a luminance changes by a prescribed value) is low, dynamic driving is liable to suffer from reduction in contrast or reduction in viewing angle. Furthermore, these conventional bistable liquid crystals do not exhibit hysteresis as shown in FIG. 1 but that shown in FIG. 2 so that they have no memory effect. Therefore, it is necessary to continue applying a voltage of .nu..sub.3 of FIG. 2 or applying a high frequency for the liquid crystal to maintain a stable response in the Sc* phase, which, in either case, entails a considerable energy loss.
Thus, the conventional electro-optic devices have many problems waiting for solution notwithstanding the strong demand for making effective use of the strong connection between an applied electric field and molecular orientation exhibited by ferroelectric liquid crystals.